1. Field of the Invention
This invention relates to transparent laminates and optical filters for displays using the same. More particularly, it relates to transparent laminates having excellent electromagnetic shielding characteristics and near-infrared blocking characteristics as well as high transparency, and to optical filters for displays which use such transparent laminates and which have excellent electromagnetic shielding characteristics, excellent near-infrared blocking characteristics, high transparency, low reflectance, good weather resistance and good environmental resistance, in combination with antiglare properties and/or anti-Newton ring properties.
2. Description of the Prior Art
With the rapid development of information in society, optoelectronic components and equipment have been markedly advanced and popularized. Among others, displays for presenting pictures have spread wide for use as television receivers and monitors for personal computers, and the like, and they are increasing in size while decreasing in thickness. In recent years, plasma displays have attracted attention because of their suitability for use in applications such as large-sized thin television receivers and monitors, and have begun to come into the market. However, on the basis of their structure and operational principle, plasma displays emit a strong leakage electromagnetic field. In recent years, the influence of a leakage electromagnetic field on the human body and other electronic equipment has come to be discussed, and it has become necessary to keep the leakage electromagnetic field emerging from plasma displays within the limits of established safety standards. Such safety standards include, for examples, the standards set up by VCCI (Voluntary Control Council for Interference by data processing equipment and electronic office machines) in Japan, and the standards set up by FCC (Federal Communication Commission) in U.S.A.
Moreover, plasma displays emit intense near-infrared radiation which may act on electronic equipment such as cordless phones and infrared remote controllers and cause malfunction thereof. The wavelengths which are of particular interest are those used in infrared remote controllers and optical communication by transmission systems, such as 820 nm, 880 nm and 980 nm. Consequently, the light in the near-infrared region of the wavelength region of 800 to 1,000 nm which emerges from plasma displays must be blocked to such a level as to cause no problem from a practical point of view.
As described above, when a plasma display is used, it is necessary to reduce the levels of the electromagnetic waves and near-infrared radiation emitted from the plasma display into the environment. Consequently, it has been investigated to provide the plasma display with an optical filter having electromagnetic shielding characteristics and near-infrared cutting-off characteristics. As a matter of course, this optical filter must be highly transparent to visible light. Moreover, since this optical filter is mounted in front of the display, it is also desirable in some cases that the optical filter has a low visible light reflectance and/or excellent antiglare properties and/or anti-Newton ring properties.
With regard to the blocking of near-infrared radiation, near-infrared absorption filters made by using near-infrared absorbing dyes have been known in the prior art, but they involve various problems. For example, cyanines used as near-infrared absorbing dyes have poor stability to light and are not suitable for practical purposes. Anthraquinones and naphthoquinones have high absorptivity in the visible light region, so that near-infrared absorption filters made by using them suffer from a reduction in visible light transmittance. Phthalocyanine dyes used as near-infrared absorbing dyes have high light resistance and efficiently absorb near-infrared radiation in the wavelength region of 700 to 800 nm. However, some phthalocyanine dyes cannot absorb near-infrared radiation having wavelengths longer than 800 nm which are used in remote controllers and the like and hence responsible for malfunction thereof. Moreover, naphthalocyanine dyes are relatively expensive. Generally, near-infrared absorbing dyes have the disadvantage that they deteriorate under the action of environmental factors such as humidity, heat and light and, therefore, the near-infrared absorption filters using them undergo changes in optical properties (e.g., near-infrared blocking ability and filter color) with the lapse of time.
Since plasma displays emit intense near-infrared radiation over a wide wavelength range, it is necessary to provide them with a near-infrared absorption filter having high absorptivity for near-infrared light in a wide wavelength range. However, in order to reduce the near-infrared transmittance to such an extent as to cause no problem, the amount of dye contained in the filter must be increased. This may cause a reduction in visible light transmittance.
In optical filters for plasma displays, the increase of members resulting from performance requirements such as electromagnetic shielding cause various problems such as an increase in cost, a reduction in visible light transmittance due to the bonding of members, and an increase in the degree of reflection at bonding interfaces. Specifically, an optical filter for plasma displays is mounted in front of a plasma display to block near-infrared radiation and electromagnetic waves emerging from the display, its low visible light transmittance may contribute to a reduction in the definition of the picture. Generally, optical filters for displays should have as high a visible light transmittance as possible. Specifically, they should have a visible light transmittance of not less than 50%, preferably not less than 60%, and more preferably not less than 70%.
In order to block a leakage electromagnetic field (or electromagnetic waves) emerging from a display, the surface of the display must be covered with an object having high electric conductivity. Generally, a grounded metallic mesh, or a synthetic resin or metallic fiber mesh coated with a metal is used to block a leakage electromagnetic field. However, this technique has the disadvantage that light from the display cannot completely be transmitted, moire fringes are produced, and a low yield causes an increase in cost. It has also been investigated to use a transparent electrically conductive film, typified by indium tin oxide (ITO), as an electromagnetic shielding layer. Such transparent electrically conductive films are usually required to have electric conductivity as expressed by a sheet resistance of not greater than 10.sup.5 .OMEGA./sq. and preferably not greater than 10.sup.3 .OMEGA./sq. Useful transparent electrically conductive films include, for example, metal films formed of gold, silver, copper, platinum, palladium and the like; oxide semiconductor films formed of indium oxide, tin(IV) oxide, zinc oxide and the like; and multilayer films formed by laminating metal films and high-refractive-index transparent films alternately. Of these, transparent electrically conductive films consisting of metal films have high electric conductivity, but fail to provide a high visible light transmittance owing to the reflection and absorption of light by the metal over a wide wavelength range. Transparent electrically conductive films consisting of oxide semiconductor films have higher transparency than those consisting of metal films, but are inferior in electric conductivity and near-infrared reflectivity.
Electromagnetic waves emerging from plasma displays are very intense, and no electromagnetic shielding material that uses a transparent electrically conductive layer formed of ITO and can block electromagnetic waves emitted by plasma displays has been available in the prior art. Moreover, neither electromagnetic shielding material that uses a transparent electrically conductive layer formed of ITO, can block electromagnetic waves emitted by plasma displays, and has such high transparency as not to impair the transparency of the displays, nor electromagnetic shielding material that additionally has near-infrared cutting-off characteristics has been available in the prior art.
In contrast, multilayer films formed by laminating metal films and high-refractive-index transparent films have excellent characteristics with respect to all of electric conductivity, near-infrared blocking characteristics and visible light transmittance, owing to the electric conductivity and optical properties possessed by metals such as silver, and the ability of the high-refractive-index transparent films to prevent the reflection of light by metals in a certain wavelength range.
A laminate comprising a multilayer film formed by laminating metal films and high-refractive-index transparent films is disclosed in Japanese Patent Publication No. 32436/1996 (JP, B2, 8-32436). However, this laminate does not have sufficient capability to block intense electromagnetic waves and near-infrared radiation emerging from plasma displays. Moreover, the use of this laminate as an optical filter for displays is not described therein.
Transparent electric conductors using thin films have the disadvantage that their film-bearing surfaces generally have poor scratch resistance. Moreover, transparent electric conductors comprising metal films or multilayer films formed by sandwiching metal films between high-refractive-index transparent films have the disadvantage that the films may undergo chemical or physical changes under the action of gases present in the environment for use. Especially when silver is used for the metal films, the silver aggregates and whitens under high-temperature and high-humidity conditions, resulting in a reduction in visibility required of optical filters for displays. Thus, when films or, in particular, multilayer films formed by laminating metal films and high-refractive-index transparent films alternately are used as transparent electrically conductive layers, these films have poor scratch resistance and environmental resistance and, therefore, must be provided with a transparent protective layer in order to protect the films.
Since an optical filter for displays is mounted on the whole surface of the display, it is required to have high transparency and low reflectivity. Moreover, anti-Newton ring properties are also required in some cases. For these reasons, an antireflection layer may be disposed on the film-bearing surface, either directly or through the medium of a transparent tacky material or adhesive. Similarly, an anti-Newton ring layer may be disposed on the film-bearing surface, either directly or through the medium of a transparent tacky material or adhesive. However, when a transparent protective layer, an antireflection layer, an anti-Newton ring layer and/or a transparent tacky material or adhesive layer are formed on the transparent electrically conductive layer of a transparent laminate, changes in the optical properties of the transparent laminate, particularly an increase in visible light reflectance and the accompanying reduction in transparency, are caused. As a result, the optical filter for displays using such a transparent laminate shows an increase in visible light reflectance and the accompanying reduction in transparency.